Replace graphql-ws with v3 subscription service

[Dreamsorcerer]

graphql-ws is dead and needs to be replaced by a subscription service in graphql-server in order to finish upgrading to v3:
graphql-python/graphql-ws#66 (comment)

This code will need to be updated for that new service and tested to check it works:

aiohttp-demos/demos/graphql-demo/graph/api/contrib.py

Lines 180 to 190 in 77791e0

	class CustomAiohttpSubscriptionServer(AiohttpSubscriptionServer):
	def get_graphql_params(self, connection_context, *args, **kwargs):
	params = super().get_graphql_params(
	connection_context,
	*args,
	**kwargs,
	)
	params.update({'context_value': connection_context.request_context})
	return params